1. Field of the Invention
The invention concerns an asynchronous digital correlator and demodulators having a correlator of this type.
This correlator enables the processing of a complex signal formed by a first carrier and a second carrier, I and Q, modulated by a pseudo-random function to spread the spectrum and modulate it by an M-ary orthogonal code. A complex signal of this type results, for example, from a PSK, MSK or APSK type modulation with spectrum spreading, and modulation by Walsh codes. The demodulation of a signal of this type makes it necessary to compress its spectrum by means of a pseudo-random function identical to the one thathas been used to spread this spectrum, and then consists in recognizing a transmitted item of data in identifying the modulating among M possible codes. These two operations can be done by means of a correlator.
2. Description of the Prior Art
At present, there are no correlators that can be used to correlate a complex signal with a reference signal on a great length, such as one of 256 chips, while having, at the same time, a structure that is simple enough to enable the making of a demodulator which can be integrated in a single integrated circuit. Prior art types of correlators are designed for frame recognition rather than for demodulation. A first type of prior art correlator device, commercially available in the form of an integrated circuit, works only on 64 chips. A second known type of correlator, commercially available in integrated circuit form, works on 256 chips and has no intermediate outputs giving partial correlation values relating to a smaller number of chips.
Known correlators have a large number of 64-chip or 256-chip correlators: they have a chain of four 64-chip correlators, or one 256-chip correlator for the carrier I; and a chain of four 64-chip correlators or one 256-chip correlator for the carrier Q, for each of the M-ary codes. Since number M may be equal to 16, the number of integrated circuits may reach 128.